In a case of cool rolling a strip material, the strip material is continuously pressed by a pair of an upper working roll and a lower working roll of a rolling machine. Due to a plastic process operation, a surface condition of the upper working roll and a surface of the lower working roll is transferred onto the front and back surfaces of the strip material, respectively. If a working roll having a defect, a crack or a chip is rolled, a harmful defect and/or design/pattern is marked on a strip material finally produced along a whole length of a coil of the strip material by synchronizing with a rotation of the upper and lower working rolls.
In the case that any defects or designs are transferred onto front and back surfaces of the rolled strip material, these defects or designs cannot be removed/disappeared even if any process is operated after rolling a produced strip material with the working roller having a defeat once in the rolling step. Such a phenomenon is a reason why a product yield is so reduced.
Thus, in order to detect a defect or a design appeared as a geometric pattern existing on front and back surfaces of a strip material, it is necessary to inspect the produced strip material periodically. In order to find out a specific rolling stand of a tandem-type cool rolling apparatus of which a working roll has a defect on its surface after finding such a defect after a pitch, a rolled strip material having a length of several meters to 15 meters is cut-out and front and back surfaces of a cut-out strip material is optically inspected.
In a general inspection method for detecting a geometric defect/pattern, the method includes a honing process, that is, the front and back surfaces of a strip material are grinded with a whetstone so as to emphasize the defects on the front and back surfaces of the rolled strip material.
Alternatively, there are other inspection methods. One is an in-line inspection method for cutting-out a strip material immediately after rolling and detecting the strip material by an inspection device arranged at a downstream side with respect to a rolling apparatus. Another is an on-line inspection method for winding up a rolled strip material, carrying out such a coil shaped strip material to an inspection device, winding out a rolled strip material and cutting out the rolled strip.
During the honing process in the inspection method, if the honing process is applied on an upper surface of the strip material stacked on an inspection table, a new defect or a design is provided on a lower surface of the strip material contacted with the inspection table due to the honing operation. It is impossible to distinguish the new defect or the new design provided in the honing process and the defect or the design provided in the rolling operation. In order to resolve such a subject, it had been disclosed an inspection method for honing a strip material without contacting the strip material with an inspection table by providing a tensile stress producing device (tensile station) at an optional position of two pairs of clamping devices. [Patent Document 1]
Alternatively, it had been disclosed an inspection method for separating two pairs of clamping devices so as to produce a tensile stress with respect to a strip material and rotating the strip material at 180 degree so as to inspect front and back surfaces of a rolled strip material. [Patent Document 2]
Patent Document 1:
    Japanese Patent Publication 2006-516484 (WO2004/069441)Patent Document 2:    Japanese Patent Publication 2008-519693 (WO2006/051239)